The present disclosure relates to a radio frequency identification (hereinafter, referred to as RFID) reader, and more particularly, to an RFID reader capable of cancelling a leakage signal.
An RFID reader separates a transmit signal from a receive signal through a directional coupler. The RFID reader transmits the transmit signal from a transmitter toward an antenna direction, and receives the receive signal of an antenna toward a receiver direction. However, because of the imperfect transmit/receive isolation of the directional coupler a leakage signal is produced in a receiver direction through the directional coupler.
FIG. 1 is a block diagram of a typical RFID system. Referring to FIG. 1, the RFID system includes an RFID reader 10 and an RFID tag 20. The RFID reader 10 transmits a signal and the RFID tag 20 responds with a corresponding signal.
The RFID reader 10 includes an antenna 11, an RF filter 12, a directional coupler 13, a transmitter 14, a frequency synthesizer 15, a receiver 16, and a digital unit 17. According to a communication protocol of a passive RFID system, when the transmitter 14 of the RFID reader 10 receives a baseband signal from the digital unit 17, a modulated signal and a continuous wave (CW) signal are transmitted alternately.
When the RFID reader 10 transmits a modulated signal, the RFID tag 20 only receives the modulated signal and does not transmit a reflected signal, such that there is no signal received by the RFID reader 10. However, when the RFID reader 10 transmits a CW signal, there is a reflected signal from the RFID tag 20, which needs to be processed by the receiver 16 of the RFID reader 10.
The RFID tag 20 absorbs a portion of a CW signal from the RFID reader 10 and reflects the remaining CW signal. The RFID reader 10 simultaneously receives a signal while transmitting a CW signal. Accordingly, the same frequency is used in the RFID reader 10 during the process of transmitting and receiving a signal.
The transmitter 14 generates a CW signal and transmits it to the directional coupler 13. When the CW signal passes through the directional coupler 13, a portion of the CW signal is transmitted to the receiver 16, and the remaining signal passes through the RF filter 12 and the antenna 11 before is transmitted to the RFID tag 20.
In the RFID reader 10, because only one antenna for transmitting and receiving a signal is used, the transmitting and receiving of the signal are separated using the directional coupler 13. That is, a transmit signal is delivered only in an antenna direction through the directional coupler 13. However, because of the imperfect transmit/receive isolation of the directional coupler 13, a leakage signal flows in the receiver direction.
The receiver 16 simultaneously receives a leakage signal SL leaked from the transmitter 14 and a RFID tag reflected signal SR from the RFID tag 20. Accordingly, it is difficult for the receiver 16 to demodulate only the RFID tag reflected signal SR from the RFID tag 20.
FIGS. 2A through 2E are graphs illustrating signal spectrums of the RFID system of FIG. 1. FIG. 2A illustrates a signal spectrum of a transmit signal 1 transmitted from the transmitter 14 of the passive RFID system of FIG. 1. FIG. 2B illustrates a signal spectrum of a leakage signal 2 leaked from the transmitter 14 of the RFID system toward the receiver 16. FIG. 2C illustrates a signal spectrum of RFID tag reflected signals 3, 4a, and 4b in the RFID tag 20 of the passive RFID system of FIG. 1. The RFID tag reflected signal includes a carrier wave component 3 and modulated components 4a and 4b including tag information.
FIG. 2D illustrates a signal spectrum of a synthesize signal combined with the leakage signal SL of FIG. 2B and the RFID tag reflected signal SR of FIG. 2C in the receiver 16 of the passive RFID system. The RFID reader 10 should extract only the modulated components 4a and 4b. However, because of a significant flow amount of the leakage signal 2, the carrier wave component 3 of a receive signal is increased. Therefore, aliasing (where spectrums of the carrier wave component 3 and the modulated components 4a and 4b overlap) occurs such that it is difficult to extract only the modulated components 4a and 4b. 
FIG. 2E illustrates a signal spectrum of a signal after filtering in the receiver 16 when there is no leakage signal in the passive RFID system of FIG. 1. FIG. 2F illustrates a signal spectrum of signals after filtering in the receiver 16 when there is a leakage signal in the passive RFID system of FIG. 1. Comparing FIG. 2E and FIG. 2F, when there is no leakage signal component, only the modulated component can be extracted after filtering, but it is impossible to extract only the modulated component when there is a leakage signal component because the modulated component and the leakage signal component are synthesized after filtering.
To resolve limitations due to a leakage signal, a method of extracting a portion of a transmit signal through a directional coupler of a transmitter has been proposed. However, this method also has limitations such as power loss of a transmit signal.